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<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <script>
      // 变量的 作用域（生效范围） 链
      {
        let age = 21;
        {
          // let age = 22;
          {
            // let age = 23;
            {
              // let age = 24;
              console.log(age); // 23
            }
          }
        }
      }
    </script>
  </body>
</html>
